Accretion Disk Atmospheres
77 Massachusetts Avenue
NE80-6009, Center for Space Research
Cambridge, MA 02139
The window on high-resolution X-ray spectroscopy opened by
Chandra and XMM-Newton is revealing the nature of the atmospheres
and coronae of accretion disks. Observations of X-ray binaries
show that dense photoionized plasmas blanket the disk. We compare
accretion disk model atmospheres with observed X-ray spectra
in order to derive ionization and density structure, opacity,
spatial distribution, elemental composition, energetics, thermal
We calculated the spectrum for a disk atmosphere surrounding
a supermassive Kerr black hole. I show the observable line emission
signature in the soft X-rays from the flow near the horizon.
This spectrum is subject to line transfer effects. Via a Monte
Carlo, we find that the line ratios will be modified after repeated
photoionizations and recombinations inside the atmosphere. Lines
such O VIII (654 eV) and C VI (367 eV) are enhanced, while others
such as Ne X (1022 eV) and O VIII RRC (871 eV) are suppressed.
We predict an anti-correlation between the Fe K fluorescence
and the recombination line intensities, traceable to the atmospheric
structure. Our results suggest that the O VIII and N VII lines
identified with XMM-Newton in MCG-6-30-15 (Branduardi-Raymont
et al., 2001) are overly bright, but may be physically tenable.